The present invention relates generally to the field of hydraulic circuits utilized in automatic transmission systems and, more particularly, to a direct replacement reverse boost valve assembly for a General Motors 4L60E (hereinafter xe2x80x9cGMxe2x80x9d) transmission that acts to increase hydraulic pump output as engine torque increases and to increase the operating range of line pressure in reverse gear.
Automatic transmission systems of the prior art have a hydraulic circuit subsystem which includes a hydraulic pump having fluid conducting passages or circuits, a valve body having fluid conducting circuits, input and exhaust ports formed within such fluid circuits, and a plurality of spool valves so-called because of their general resemblance to sewing-thread type spools. Such valves are comprised of cylindrical pistons having control diameters or lands formed thereon, which alternately open and close the ports to the fluid circuits to regulate the flow and pressure of automatic transmission fluid (hereinafter xe2x80x9cATFxe2x80x9d) within the fluid circuits to actuate different components of the transmission. It will be understood that in describing hydraulic circuits, ATF usually changes names when it passes through an orifice or control valve in a specific fluid circuit.
The reverse boost valve is a spool-type valve, which is disposed within a mating sleeve and is installed within the hydraulic pump of the transmission. The reverse boost valve functions in combination with the pressure regulator valve to increase line pressure as engine torque increases. It also functions to increase the operating range of line pressure when the transmission is in reverse gear. Torque signal fluid pressure (this fluid pressure is proportional to engine torque) moves the reverse boost valve piston against the isolator spring located in the hydraulic pump assembly. The isolator spring then transfers the force of the torque signal fluid pressure to the pressure regulator valve, which in turn raises line pressure. Thus, line pressure increases as throttle position and engine torque increase. Reverse input fluid pressure acting on the reverse boost valve also increases the operating range of line pressure when the transmission is in reverse gear.
Line pressure leakage can eventually develop due to the constant oscillation and mechanical wear of the original equipment manufacture (hereinafter xe2x80x9cOEMxe2x80x9d) reverse boost valve piston within its mating sleeve. When this occurs ATF that enters the torque signal orifice in the valve sleeve leaks past the valve piston and exhausts through the reverse input orifice resulting in poor line pressure rise causing clutch/band failure and/or poor shift quality. Similarly, fluid may leak past the valve piston in the opposite direction when fluid enters the valve chamber through the reverse input orifice in reverse gear and escapes via the torque signal orifice.
Thus, the present replacement reverse boost valve has been developed to provide a solution to these problems and other shortcomings of the prior art valve assembly.
Accordingly, the present invention provides a replacement reverse boost valve assembly wherein the valve piston and mating valve sleeve are manufactured from a high quality aluminum material specifically designed to resist wear. In addition, a wear resistant, hard anodize coating is applied to the valve piston to provide a low coefficient of friction between the mating valve surfaces. The present reverse boost valve assembly is provided in both standard OEM (0.470xe2x80x3 spool diameters) and oversize (0.490xe2x80x3 spool diameters) with or without O-ring seals. The O-ring seals provide additional protection against leakage through the pump body and maintain pressure in the hydraulic circuits that supply the valve. The variable size spool diameters and optional seal configurations provide for interchangeability of these alternative embodiments in the hydraulic pump to provide an increased rate of line pressure rise for a particular transmission or vehicle use.